Known units for gumming sheet material normally feature mechanical, and in particular roller or spray, gum applying devices, which, though highly efficient, involve several drawbacks.
In particular, roller devices operate at relatively slow speed, well below the requirements of modern machinery, particularly modern packing machines, and, by featuring a gumming roller rotating partly immersed in a bath of gum, are particularly "dirty" and therefore require frequent cleaning. Moreover, during machine stoppages, the bath of gum tends to dry by remaining in contact with the air with no provision for stirring the gum.
On the other hand, spray devices are relatively expensive and unreliable, due to frequent clogging of the spray nozzles.